Fix: fmt's {:#04X} yields '0X00', not '0x0000'

Technically the 0X vs 0x is not a big problem, just not pretty. However, the
length also including the 0x results in unexpected behaviour, so it probably
better to not use it.

src/newgrf_generic.cpp

@@ -109,7 +109,7 @@ void ResetGenericCallbacks()
 void AddGenericCallback(uint8 feature, const GRFFile *file, const SpriteGroup *group)
 {
 	if (feature >= lengthof(_gcl)) {
-		GrfMsg(5, "AddGenericCallback: Unsupported feature {:#02X}", feature);
+		GrfMsg(5, "AddGenericCallback: Unsupported feature 0x{:02X}", feature);
 		return;
 	}